ASP.NET - The specified network password is not correct

I also had an issue with a pfx file, problem was, it was exported using AES256-SHA256 encryption, which threw the same exception as in the question. According to this, AES256-SHA256 is only supported on Windows 10 1703, Windows Server 2016 and above. Changing to TripleDES-SHA1 'solved' the issue.

Try this:

new X509Certificate2(Path, "", X509KeyStorageFlags.MachineKeySet); 

It appears that the X509Certificate2 constructor tries to access the private key store of the local user (even when loading a PFX and the private key is in the PFX). With asp.net, the user profile typically isn't loaded, so the user key store doesn't exist. Specifying MachineKeySet tells the constructor to look at the Local Computer key store which always exists.

I had this issue in with a PFX certificate file that had a 40 character password set, Windows allowed me to export it and also import it, but it couldn't be used in C# code, so I changed to a 30 character password and it works.